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24 KiB
ReStructuredText
609 lines
24 KiB
ReStructuredText
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Terminology
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===========
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This section introduces terminology used in this document.
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*Repository*: All data produced during a backup is sent to and stored in
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a repository in a structured form, for example in a file system
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hierarchy with several subdirectories. A repository implementation must
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be able to fulfill a number of operations, e.g. list the contents.
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*Blob*: A Blob combines a number of data bytes with identifying
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information like the SHA-256 hash of the data and its length.
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*Pack*: A Pack combines one or more Blobs, e.g. in a single file.
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*Snapshot*: A Snapshot stands for the state of a file or directory that
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has been backed up at some point in time. The state here means the
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content and meta data like the name and modification time for the file
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or the directory and its contents.
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*Storage ID*: A storage ID is the SHA-256 hash of the content stored in
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the repository. This ID is required in order to load the file from the
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repository.
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Repository Format
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=================
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All data is stored in a restic repository. A repository is able to store
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data of several different types, which can later be requested based on
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an ID. This so-called "storage ID" is the SHA-256 hash of the content of
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a file. All files in a repository are only written once and never
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modified afterwards. This allows accessing and even writing to the
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repository with multiple clients in parallel. Only the ``prune`` operation
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removes data from the repository.
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Repositories consist of several directories and a top-level file called
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``config``. For all other files stored in the repository, the name for
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the file is the lower case hexadecimal representation of the storage ID,
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which is the SHA-256 hash of the file's contents. This allows for easy
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verification of files for accidental modifications, like disk read
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errors, by simply running the program ``sha256sum`` on the file and
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comparing its output to the file name. If the prefix of a filename is
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unique amongst all the other files in the same directory, the prefix may
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be used instead of the complete filename.
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Apart from the files stored within the ``keys`` directory, all files are
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encrypted with AES-256 in counter mode (CTR). The integrity of the
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encrypted data is secured by a Poly1305-AES message authentication code
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(sometimes also referred to as a "signature").
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In the first 16 bytes of each encrypted file the initialisation vector
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(IV) is stored. It is followed by the encrypted data and completed by
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the 16 byte MAC. The format is: ``IV || CIPHERTEXT || MAC``. The
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complete encryption overhead is 32 bytes. For each file, a new random IV
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is selected.
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The file ``config`` is encrypted this way and contains a JSON document
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like the following:
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.. code:: json
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{
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"version": 1,
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"id": "5956a3f67a6230d4a92cefb29529f10196c7d92582ec305fd71ff6d331d6271b",
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"chunker_polynomial": "25b468838dcb75"
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}
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After decryption, restic first checks that the version field contains a
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version number that it understands, otherwise it aborts. At the moment,
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the version is expected to be 1. The field ``id`` holds a unique ID
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which consists of 32 random bytes, encoded in hexadecimal. This uniquely
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identifies the repository, regardless if it is accessed via SFTP or
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locally. The field ``chunker_polynomial`` contains a parameter that is
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used for splitting large files into smaller chunks (see below).
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Repository Layout
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-----------------
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The ``local`` and ``sftp`` backends are implemented using files and
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directories stored in a file system. The directory layout is the same
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for both backend types.
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The basic layout of a repository stored in a ``local`` or ``sftp``
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backend is shown here:
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::
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/tmp/restic-repo
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├── config
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├── data
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│ ├── 21
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│ │ └── 2159dd48f8a24f33c307b750592773f8b71ff8d11452132a7b2e2a6a01611be1
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│ ├── 32
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│ │ └── 32ea976bc30771cebad8285cd99120ac8786f9ffd42141d452458089985043a5
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│ ├── 59
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│ │ └── 59fe4bcde59bd6222eba87795e35a90d82cd2f138a27b6835032b7b58173a426
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│ ├── 73
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│ │ └── 73d04e6125cf3c28a299cc2f3cca3b78ceac396e4fcf9575e34536b26782413c
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│ [...]
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├── index
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│ ├── c38f5fb68307c6a3e3aa945d556e325dc38f5fb68307c6a3e3aa945d556e325d
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│ └── ca171b1b7394d90d330b265d90f506f9984043b342525f019788f97e745c71fd
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├── keys
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│ └── b02de829beeb3c01a63e6b25cbd421a98fef144f03b9a02e46eff9e2ca3f0bd7
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├── locks
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├── snapshots
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│ └── 22a5af1bdc6e616f8a29579458c49627e01b32210d09adb288d1ecda7c5711ec
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└── tmp
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A local repository can be initialized with the ``restic init`` command,
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e.g.:
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.. code-block:: console
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$ restic -r /tmp/restic-repo init
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The local and sftp backends will auto-detect and accept all layouts described
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in the following sections, so that remote repositories mounted locally e.g. via
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fuse can be accessed. The layout auto-detection can be overridden by specifying
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the option ``-o local.layout=default``, valid values are ``default`` and
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``s3legacy``. The option for the sftp backend is named ``sftp.layout``, for the
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s3 backend ``s3.layout``.
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S3 Legacy Layout
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----------------
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Unfortunately during development the AWS S3 backend uses slightly different
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paths (directory names use singular instead of plural for ``key``,
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``lock``, and ``snapshot`` files), and the data files are stored directly below
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the ``data`` directory. The S3 Legacy repository layout looks like this:
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::
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/config
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/data
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├── 2159dd48f8a24f33c307b750592773f8b71ff8d11452132a7b2e2a6a01611be1
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├── 32ea976bc30771cebad8285cd99120ac8786f9ffd42141d452458089985043a5
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├── 59fe4bcde59bd6222eba87795e35a90d82cd2f138a27b6835032b7b58173a426
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├── 73d04e6125cf3c28a299cc2f3cca3b78ceac396e4fcf9575e34536b26782413c
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[...]
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/index
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├── c38f5fb68307c6a3e3aa945d556e325dc38f5fb68307c6a3e3aa945d556e325d
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└── ca171b1b7394d90d330b265d90f506f9984043b342525f019788f97e745c71fd
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/key
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└── b02de829beeb3c01a63e6b25cbd421a98fef144f03b9a02e46eff9e2ca3f0bd7
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/lock
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/snapshot
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└── 22a5af1bdc6e616f8a29579458c49627e01b32210d09adb288d1ecda7c5711ec
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The S3 backend understands and accepts both forms, new backends are
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always created with the default layout for compatibility reasons.
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Pack Format
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===========
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All files in the repository except Key and Pack files just contain raw
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data, stored as ``IV || Ciphertext || MAC``. Pack files may contain one
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or more Blobs of data.
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A Pack's structure is as follows:
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::
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EncryptedBlob1 || ... || EncryptedBlobN || EncryptedHeader || Header_Length
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At the end of the Pack file is a header, which describes the content.
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The header is encrypted and authenticated. ``Header_Length`` is the
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length of the encrypted header encoded as a four byte integer in
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little-endian encoding. Placing the header at the end of a file allows
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writing the blobs in a continuous stream as soon as they are read during
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the backup phase. This reduces code complexity and avoids having to
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re-write a file once the pack is complete and the content and length of
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the header is known.
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All the blobs (``EncryptedBlob1``, ``EncryptedBlobN`` etc.) are
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authenticated and encrypted independently. This enables repository
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reorganisation without having to touch the encrypted Blobs. In addition
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it also allows efficient indexing, for only the header needs to be read
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in order to find out which Blobs are contained in the Pack. Since the
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header is authenticated, authenticity of the header can be checked
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without having to read the complete Pack.
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After decryption, a Pack's header consists of the following elements:
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::
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Type_Blob1 || Length(EncryptedBlob1) || Hash(Plaintext_Blob1) ||
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[...]
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Type_BlobN || Length(EncryptedBlobN) || Hash(Plaintext_Blobn) ||
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This is enough to calculate the offsets for all the Blobs in the Pack.
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Length is the length of a Blob as a four byte integer in little-endian
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format. The type field is a one byte field and labels the content of a
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blob according to the following table:
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+--------+-----------+
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| Type | Meaning |
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+========+===========+
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| 0 | data |
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+--------+-----------+
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| 1 | tree |
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+--------+-----------+
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All other types are invalid, more types may be added in the future.
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For reconstructing the index or parsing a pack without an index, first
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the last four bytes must be read in order to find the length of the
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header. Afterwards, the header can be read and parsed, which yields all
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plaintext hashes, types, offsets and lengths of all included blobs.
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Indexing
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========
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Index files contain information about Data and Tree Blobs and the Packs
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they are contained in and store this information in the repository. When
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the local cached index is not accessible any more, the index files can
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be downloaded and used to reconstruct the index. The files are encrypted
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and authenticated like Data and Tree Blobs, so the outer structure is
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``IV || Ciphertext || MAC`` again. The plaintext consists of a JSON
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document like the following:
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.. code:: json
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{
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"supersedes": [
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"ed54ae36197f4745ebc4b54d10e0f623eaaaedd03013eb7ae90df881b7781452"
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],
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"packs": [
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{
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"id": "73d04e6125cf3c28a299cc2f3cca3b78ceac396e4fcf9575e34536b26782413c",
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"blobs": [
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{
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"id": "3ec79977ef0cf5de7b08cd12b874cd0f62bbaf7f07f3497a5b1bbcc8cb39b1ce",
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"type": "data",
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"offset": 0,
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"length": 25
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},{
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"id": "9ccb846e60d90d4eb915848add7aa7ea1e4bbabfc60e573db9f7bfb2789afbae",
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"type": "tree",
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"offset": 38,
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"length": 100
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},
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{
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"id": "d3dc577b4ffd38cc4b32122cabf8655a0223ed22edfd93b353dc0c3f2b0fdf66",
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"type": "data",
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"offset": 150,
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"length": 123
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}
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]
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}, [...]
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]
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}
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This JSON document lists Packs and the blobs contained therein. In this
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example, the Pack ``73d04e61`` contains two data Blobs and one Tree
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blob, the plaintext hashes are listed afterwards.
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The field ``supersedes`` lists the storage IDs of index files that have
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been replaced with the current index file. This happens when index files
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are repacked, for example when old snapshots are removed and Packs are
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recombined.
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There may be an arbitrary number of index files, containing information
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on non-disjoint sets of Packs. The number of packs described in a single
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file is chosen so that the file size is kept below 8 MiB.
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Keys, Encryption and MAC
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========================
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All data stored by restic in the repository is encrypted with AES-256 in
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counter mode and authenticated using Poly1305-AES. For encrypting new
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data first 16 bytes are read from a cryptographically secure
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pseudorandom number generator as a random nonce. This is used both as
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the IV for counter mode and the nonce for Poly1305. This operation needs
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three keys: A 32 byte for AES-256 for encryption, a 16 byte AES key and
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a 16 byte key for Poly1305. For details see the original paper `The
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Poly1305-AES message-authentication
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code <https://cr.yp.to/mac/poly1305-20050329.pdf>`__ by Dan Bernstein.
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The data is then encrypted with AES-256 and afterwards a message
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authentication code (MAC) is computed over the ciphertext, everything is
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then stored as IV \|\| CIPHERTEXT \|\| MAC.
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The directory ``keys`` contains key files. These are simple JSON
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documents which contain all data that is needed to derive the
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repository's master encryption and message authentication keys from a
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user's password. The JSON document from the repository can be
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pretty-printed for example by using the Python module ``json``
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(shortened to increase readability):
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::
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$ python -mjson.tool /tmp/restic-repo/keys/b02de82*
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{
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"hostname": "kasimir",
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"username": "fd0"
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"kdf": "scrypt",
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"N": 65536,
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"r": 8,
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"p": 1,
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"created": "2015-01-02T18:10:13.48307196+01:00",
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"data": "tGwYeKoM0C4j4/9DFrVEmMGAldvEn/+iKC3te/QE/6ox/V4qz58FUOgMa0Bb1cIJ6asrypCx/Ti/pRXCPHLDkIJbNYd2ybC+fLhFIJVLCvkMS+trdywsUkglUbTbi+7+Ldsul5jpAj9vTZ25ajDc+4FKtWEcCWL5ICAOoTAxnPgT+Lh8ByGQBH6KbdWabqamLzTRWxePFoYuxa7yXgmj9A==",
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"salt": "uW4fEI1+IOzj7ED9mVor+yTSJFd68DGlGOeLgJELYsTU5ikhG/83/+jGd4KKAaQdSrsfzrdOhAMftTSih5Ux6w==",
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}
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When the repository is opened by restic, the user is prompted for the
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repository password. This is then used with ``scrypt``, a key derivation
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function (KDF), and the supplied parameters (``N``, ``r``, ``p`` and
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``salt``) to derive 64 key bytes. The first 32 bytes are used as the
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encryption key (for AES-256) and the last 32 bytes are used as the
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message authentication key (for Poly1305-AES). These last 32 bytes are
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divided into a 16 byte AES key ``k`` followed by 16 bytes of secret key
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``r``. The key ``r`` is then masked for use with Poly1305 (see the paper
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for details).
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Those keys are used to authenticate and decrypt the bytes contained in
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the JSON field ``data`` with AES-256 and Poly1305-AES as if they were
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any other blob (after removing the Base64 encoding). If the
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password is incorrect or the key file has been tampered with, the
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computed MAC will not match the last 16 bytes of the data, and restic
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exits with an error. Otherwise, the data yields a JSON document
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which contains the master encryption and message authentication keys for
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this repository (encoded in Base64). The command
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``restic cat masterkey`` can be used as follows to decrypt and
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pretty-print the master key:
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.. code-block:: console
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$ restic -r /tmp/restic-repo cat masterkey
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{
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"mac": {
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"k": "evFWd9wWlndL9jc501268g==",
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"r": "E9eEDnSJZgqwTOkDtOp+Dw=="
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},
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"encrypt": "UQCqa0lKZ94PygPxMRqkePTZnHRYh1k1pX2k2lM2v3Q=",
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}
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All data in the repository is encrypted and authenticated with these
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master keys. For encryption, the AES-256 algorithm in Counter mode is
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used. For message authentication, Poly1305-AES is used as described
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above.
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A repository can have several different passwords, with a key file for
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each. This way, the password can be changed without having to re-encrypt
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all data.
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Snapshots
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=========
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A snapshot represents a directory with all files and sub-directories at
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a given point in time. For each backup that is made, a new snapshot is
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created. A snapshot is a JSON document that is stored in an encrypted
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file below the directory ``snapshots`` in the repository. The filename
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is the storage ID. This string is unique and used within restic to
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uniquely identify a snapshot.
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The command ``restic cat snapshot`` can be used as follows to decrypt
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and pretty-print the contents of a snapshot file:
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.. code-block:: console
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$ restic -r /tmp/restic-repo cat snapshot 251c2e58
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enter password for repository:
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{
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"time": "2015-01-02T18:10:50.895208559+01:00",
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"tree": "2da81727b6585232894cfbb8f8bdab8d1eccd3d8f7c92bc934d62e62e618ffdf",
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"dir": "/tmp/testdata",
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"hostname": "kasimir",
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"username": "fd0",
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"uid": 1000,
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"gid": 100,
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"tags": [
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"NL"
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]
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}
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Here it can be seen that this snapshot represents the contents of the
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directory ``/tmp/testdata``. The most important field is ``tree``. When
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the meta data (e.g. the tags) of a snapshot change, the snapshot needs
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to be re-encrypted and saved. This will change the storage ID, so in
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order to relate these seemingly different snapshots, a field
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``original`` is introduced which contains the ID of the original
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snapshot, e.g. after adding the tag ``DE`` to the snapshot above it
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becomes:
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.. code-block:: console
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$ restic -r /tmp/restic-repo cat snapshot 22a5af1b
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enter password for repository:
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{
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"time": "2015-01-02T18:10:50.895208559+01:00",
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"tree": "2da81727b6585232894cfbb8f8bdab8d1eccd3d8f7c92bc934d62e62e618ffdf",
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"dir": "/tmp/testdata",
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"hostname": "kasimir",
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"username": "fd0",
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"uid": 1000,
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"gid": 100,
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"tags": [
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"NL",
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"DE"
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],
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"original": "251c2e5841355f743f9d4ffd3260bee765acee40a6229857e32b60446991b837"
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}
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Once introduced, the ``original`` field is not modified when the
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snapshot's meta data is changed again.
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All content within a restic repository is referenced according to its
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SHA-256 hash. Before saving, each file is split into variable sized
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Blobs of data. The SHA-256 hashes of all Blobs are saved in an ordered
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list which then represents the content of the file.
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In order to relate these plaintext hashes to the actual location within
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a Pack file , an index is used. If the index is not available, the
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header of all data Blobs can be read.
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Trees and Data
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==============
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A snapshot references a tree by the SHA-256 hash of the JSON string
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representation of its contents. Trees and data are saved in pack files
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in a subdirectory of the directory ``data``.
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The command ``restic cat blob`` can be used to inspect the tree
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referenced above (piping the output of the command to ``jq .`` so that
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the JSON is indented):
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.. code-block:: console
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$ restic -r /tmp/restic-repo cat blob 2da81727b6585232894cfbb8f8bdab8d1eccd3d8f7c92bc934d62e62e618ffdf | jq .
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enter password for repository:
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{
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"nodes": [
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{
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"name": "testdata",
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"type": "dir",
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"mode": 493,
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"mtime": "2014-12-22T14:47:59.912418701+01:00",
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"atime": "2014-12-06T17:49:21.748468803+01:00",
|
|
"ctime": "2014-12-22T14:47:59.912418701+01:00",
|
|
"uid": 1000,
|
|
"gid": 100,
|
|
"user": "fd0",
|
|
"inode": 409704562,
|
|
"content": null,
|
|
"subtree": "b26e315b0988ddcd1cee64c351d13a100fedbc9fdbb144a67d1b765ab280b4dc"
|
|
}
|
|
]
|
|
}
|
|
|
|
A tree contains a list of entries (in the field ``nodes``) which contain
|
|
meta data like a name and timestamps. When the entry references a
|
|
directory, the field ``subtree`` contains the plain text ID of another
|
|
tree object.
|
|
|
|
When the command ``restic cat blob`` is used, the plaintext ID is needed
|
|
to print a tree. The tree referenced above can be dumped as follows:
|
|
|
|
.. code-block:: console
|
|
|
|
$ restic -r /tmp/restic-repo cat blob b26e315b0988ddcd1cee64c351d13a100fedbc9fdbb144a67d1b765ab280b4dc
|
|
enter password for repository:
|
|
{
|
|
"nodes": [
|
|
{
|
|
"name": "testfile",
|
|
"type": "file",
|
|
"mode": 420,
|
|
"mtime": "2014-12-06T17:50:23.34513538+01:00",
|
|
"atime": "2014-12-06T17:50:23.338468713+01:00",
|
|
"ctime": "2014-12-06T17:50:23.34513538+01:00",
|
|
"uid": 1000,
|
|
"gid": 100,
|
|
"user": "fd0",
|
|
"inode": 416863351,
|
|
"size": 1234,
|
|
"links": 1,
|
|
"content": [
|
|
"50f77b3b4291e8411a027b9f9b9e64658181cc676ce6ba9958b95f268cb1109d"
|
|
]
|
|
},
|
|
[...]
|
|
]
|
|
}
|
|
|
|
This tree contains a file entry. This time, the ``subtree`` field is not
|
|
present and the ``content`` field contains a list with one plain text
|
|
SHA-256 hash.
|
|
|
|
The command ``restic cat blob`` can also be used to extract and decrypt
|
|
data given a plaintext ID, e.g. for the data mentioned above:
|
|
|
|
.. code-block:: console
|
|
|
|
$ restic -r /tmp/restic-repo cat blob 50f77b3b4291e8411a027b9f9b9e64658181cc676ce6ba9958b95f268cb1109d | sha256sum
|
|
enter password for repository:
|
|
50f77b3b4291e8411a027b9f9b9e64658181cc676ce6ba9958b95f268cb1109d -
|
|
|
|
As can be seen from the output of the program ``sha256sum``, the hash
|
|
matches the plaintext hash from the map included in the tree above, so
|
|
the correct data has been returned.
|
|
|
|
Locks
|
|
=====
|
|
|
|
The restic repository structure is designed in a way that allows
|
|
parallel access of multiple instance of restic and even parallel writes.
|
|
However, there are some functions that work more efficient or even
|
|
require exclusive access of the repository. In order to implement these
|
|
functions, restic processes are required to create a lock on the
|
|
repository before doing anything.
|
|
|
|
Locks come in two types: Exclusive and non-exclusive locks. At most one
|
|
process can have an exclusive lock on the repository, and during that
|
|
time there must not be any other locks (exclusive and non-exclusive).
|
|
There may be multiple non-exclusive locks in parallel.
|
|
|
|
A lock is a file in the subdir ``locks`` whose filename is the storage
|
|
ID of the contents. It is encrypted and authenticated the same way as
|
|
other files in the repository and contains the following JSON structure:
|
|
|
|
.. code:: json
|
|
|
|
{
|
|
"time": "2015-06-27T12:18:51.759239612+02:00",
|
|
"exclusive": false,
|
|
"hostname": "kasimir",
|
|
"username": "fd0",
|
|
"pid": 13607,
|
|
"uid": 1000,
|
|
"gid": 100
|
|
}
|
|
|
|
The field ``exclusive`` defines the type of lock. When a new lock is to
|
|
be created, restic checks all locks in the repository. When a lock is
|
|
found, it is tested if the lock is stale, which is the case for locks
|
|
with timestamps older than 30 minutes. If the lock was created on the
|
|
same machine, even for younger locks it is tested whether the process is
|
|
still alive by sending a signal to it. If that fails, restic assumes
|
|
that the process is dead and considers the lock to be stale.
|
|
|
|
When a new lock is to be created and no other conflicting locks are
|
|
detected, restic creates a new lock, waits, and checks if other locks
|
|
appeared in the repository. Depending on the type of the other locks and
|
|
the lock to be created, restic either continues or fails.
|
|
|
|
Backups and Deduplication
|
|
=========================
|
|
|
|
For creating a backup, restic scans the source directory for all files,
|
|
sub-directories and other entries. The data from each file is split into
|
|
variable length Blobs cut at offsets defined by a sliding window of 64
|
|
byte. The implementation uses Rabin Fingerprints for implementing this
|
|
Content Defined Chunking (CDC). An irreducible polynomial is selected at
|
|
random and saved in the file ``config`` when a repository is
|
|
initialized, so that watermark attacks are much harder.
|
|
|
|
Files smaller than 512 KiB are not split, Blobs are of 512 KiB to 8 MiB
|
|
in size. The implementation aims for 1 MiB Blob size on average.
|
|
|
|
For modified files, only modified Blobs have to be saved in a subsequent
|
|
backup. This even works if bytes are inserted or removed at arbitrary
|
|
positions within the file.
|
|
|
|
Threat Model
|
|
============
|
|
|
|
The design goals for restic include being able to securely store backups
|
|
in a location that is not completely trusted, e.g. a shared system where
|
|
others can potentially access the files or (in the case of the system
|
|
administrator) even modify or delete them.
|
|
|
|
General assumptions:
|
|
|
|
- The host system a backup is created on is trusted. This is the most
|
|
basic requirement, and essential for creating trustworthy backups.
|
|
|
|
The restic backup program guarantees the following:
|
|
|
|
- Accessing the unencrypted content of stored files and metadata should
|
|
not be possible without a password for the repository. Everything
|
|
except the metadata included for informational purposes in the key
|
|
files is encrypted and authenticated.
|
|
|
|
- Modifications (intentional or unintentional) can be detected
|
|
automatically on several layers:
|
|
|
|
1. For all accesses of data stored in the repository it is checked
|
|
whether the cryptographic hash of the contents matches the storage
|
|
ID (the file's name). This way, modifications (bad RAM, broken
|
|
harddisk) can be detected easily.
|
|
|
|
2. Before decrypting any data, the MAC on the encrypted data is
|
|
checked. If there has been a modification, the MAC check will
|
|
fail. This step happens even before the data is decrypted, so data
|
|
that has been tampered with is not decrypted at all.
|
|
|
|
However, the restic backup program is not designed to protect against
|
|
attackers deleting files at the storage location. There is nothing that
|
|
can be done about this. If this needs to be guaranteed, get a secure
|
|
location without any access from third parties. If you assume that
|
|
attackers have write access to your files at the storage location,
|
|
attackers are able to figure out (e.g. based on the timestamps of the
|
|
stored files) which files belong to what snapshot. When only these files
|
|
are deleted, the particular snapshot vanished and all snapshots
|
|
depending on data that has been added in the snapshot cannot be restored
|
|
completely. Restic is not designed to detect this attack.
|
|
|